A latex agglutination test was highly successful in detecting Candida antigen in serum from patients with severe desseminated candidiasis, provided that the antigen was first dissociated from high molecular weight complexes using protease and heat. Of 21 patients with disseminated candidiasis, 81% had antigen in at least one serum, titers ranging 1:4 to 1:64. Serum from patients with less severe clinical forms of candidiasis were infrequently positive. False positives were rare. The test appears promising as an aid to rapid diagnosis. Patients cured from cryptococcosis but otherwise normal are known from our previous work to be selectively tolerant to immunization by cryptococcal polysaccharide. This tolerance was recently shown in peripheral blood cells cultured in vitro, even purified B cells, free of potential T cell suppression. A subset of these patients was not completely tolerant but responded to immunization by anticryptococcal antibody only of the IgA class. Peripheral blood monocytes from patients with acquired immune deficiency syndrome (AIDS) had unimpaired ability to kill fungal cells from Aspergillus, Cryptococcus and Thermoascus.